Ask Away: Can I Use a Rectangular Port Intake with Big Oval Port Heads?

I am thinking of using a big block Chevy rectangle port intake manifold on a set of aluminum Edelbrock heads that have big oval ports. Will that be okay or will there be a big horsepower loss? I’ve read that you shouldn’t do this because the engine will suffer a big power loss. I was going to disguise the heads to look like stock and I have the perfect, original factory rectangle port intake manifold (complete with the Winters snowflake) but the “wrong” heads. Can I still do this?

–Ron A., Dallas, TX

Jeff Smith: The typical answer to this question is that while the manifold will physically bolt on, this is not a good idea because of “reversion” problems. This is the standard answer, and frankly it ignores the reality of what is really going on inside an engine. This standard answer is often justified based on experience with flow benches where if the tester does not use a tapered or radius inlet around the intake port, flow numbers will suffer. This is absolutely true and dramatic flow reductions are often the result. The problem with using flow numbers to “prove” why flow will be hurt in the engine is that flow benches use much lower pressure differentials to measure airflow.

The truth is that intake manifold pressures are much greater than the industry standard 28 inches of water test depression used in flow benches. The most important concern is the way flow velocity is distributed inside a port. If you want a simple, object lesson in airflow, do this basic airflow demonstration the next time you are travelling in a car. With the window down, place your hand out in the airstream about a foot away from the door while travelling at 30-plus miles-per-hour. With your fingers pointed toward the direction of airflow, note the amount of resistance you feel. Next, slowly move your hand closer to, but not on, the door skin. What you will feel is that as you move closer to the body, the air velocity reduces. As you move your hand away from the body of the car, resistance will increase. This is called boundary layer flow. As air travels through a passage like an intake port, air velocity concentrates in the center of the port. If we were to measure air speed from the center of the intake runner and move toward the runner wall, we will notice a linear drop in velocity until we reach the wall. At the port wall, the air velocity will be zero.

[pullquote align=”left|center|right” textalign=”left|center|right” width=”30%”]…a slight mismatch of the intake manifold flange to the head should not produce a sizable loss in horsepower or torque.[/pullquote]

Applying this information to our intake manifold port connection would indicate that a slight mismatch of the intake manifold flange to the head should not produce a sizable loss in horsepower or torque. We tested this idea on a gnarly, 434-cubic-inch small block Chevy making 670 horsepower. We had two nearly identical manifolds with the second that was machined to a larger port exit to be used with larger intake port heads. Before the test, the expectation was evenly split between those who thought the engine would lose power and those who thought it would not make a difference. Based on the direction this answer is going, you’d be right if you bet that we saw no difference in either the torque curve or the peak horsepower point. The power numbers were within one ft.-lb. and one horsepower of being the same and in a couple of rpm points within the test—the mismatched intake manifold was better!

Now having stated this, we should note that there are limits to this point. If the port is smaller than the intake port flange, then the mismatch will be inconsequential. Of course, we also have to question why you would want to run a manifold with smaller port openings that represent a restriction since this could easily be sacrificing power by using a manifold that is too small. The more important question to address is when the port mismatch leaves a step or ledge between a too-large intake manifold exit port and a smaller cylinder head intake port opening. If we had to venture out onto a defined ledge, a mismatch of 1/8-inch or less is not going to cause a problem. A wider mismatch where the ledge protrudes in 1/4-inch or more, this might be significant. But if so, this may also have more to do with the mismatch of parts for the engine rather than the ledge itself. A very large intake port feeding a smaller cylinder head likely will suffer from low inlet velocity and that could be the real reason the power would be reduced. But often, these questions come up because of intake manifold choices that are based on other concerns besides torque and horsepower.

We should also address the use of the term “reversion,” which is often used as a reason why this mismatch is not a good idea. Reversion in an internal combustion engine is a somewhat generalized term for reverse—or upstream—movement of gases in the intake port and intake manifold. This can be caused by several different factors that occur in all engines. But before we get to that, we need to define a couple of processes that occur in our engine. Overlap is the term used to describe the situation where both the intake and exhaust valves are open simultaneously. At various times, this situation has been defined as the “Fifth Cycle” in the four stroke cycle of intake, compression, combustion, and exhaust. Ed Iskenderian was the first to use this term in ads for his Isky cams back in the ‘60s!

Basic illustration of overlap.

The Fifth Cycle term is used to emphasize just how important overlap is to engine performance. If there is enough interest, we can get deeper into that in a later column. If you look at a typical graph of the intake and exhaust lobe lift curves, you will see a small triangle area where the exhaust valve is just about closed and the intake valve has just started to open. This overlap period exposes the intake manifold to residual exhaust pressure present in the cylinder. If the exhaust pressure is greater than the intake manifold, then you can see that the exhaust gas can easily travel into the intake manifold before either the exhaust valve closes or the pressure equalizes. This exhaust pressure reduces intake manifold vacuum and is the major contributor to the classic lumpy idle sound that many enthusiasts cherish.

This is a more accurate definition of reversion. But reversion is often used as a term when restrictions to flow are discussed, although it should be clear that using the term reversion to explain a restriction to flow in the intake manifold isn’t really an accurate use of the term. Or maybe, we’re entirely too focused on how engines operate! That could also be true!

Thanks for your great question!

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Author: Jeff Smith
Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.